Luminescent device

ABSTRACT

A luminescent device in accordance with the present invention includes an anode, a cathode, and an organic layer containing at least one organic compound provided therebetween. The work function of the anode (Wf(anode)) and the Fermi level of the organic layer (Ef(anode)) being in contact with the anode satisfies the following equation (I):  
     Ef(anode)− 0.2 ≦Wf(anode)≦Ef(anode)+ 0.2 [eV]  (I)

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to luminescent devices which have aluminescent layer containing a luminescent material and directly convertapplied electrical energy into optical energy. In particular, thepresent invention relates to a thin, light and solid luminescent devicewith a large luminescent area and high resolution, enabling high-speedoperation. Such a luminescent device is quite different fromconventional incandescent lamps, fluorescent lamps, and light emittingdiodes (LEDs), and can be used for electroluminescence panels which areexpected as devices satisfying advanced needs in industrial fields.

[0003] 2. Description of the Related Art

[0004] Pope et al., first discovered an electroluminescence (EL) of anorganic material, that is, single-crystal anthracene in 1963 (J. Chem.Phys., 38, 2042 (1963)). Helfinch and Schneider succeeded withobservation of relatively strong EL in an injection EL materialcontaining a solution system having a high injection efficiency in 1965(Phys. Rev. Lett., 14, 229 (1965)). Many studies of organic luminescentmaterials containing conjugated organic hosts and conjugated organicactivators having condensed benzene rings have been disclosed in U.S.Pat. Nos. 3,172,862, 3,173,050, and 3,710,167; J. Chem. Phys., 44, 2902(1966); J. Chem. Phys., 58, 1542 (1973); and Chem. Phys. Lett., 36, 345(1975). Examples of disclosed organic hosts include naphthalene,anthracene, phenanthrene, tetracene, pyrene, benzpyrene, chrysene,picene, carbazole, fluorene, biphenyl, terphenyl, triphenylene oxide,dihalobiphenyl, trans-stilbene, and 1,4-diphenylbutadiene. Examples ofdisclosed activators include anthracene, tetracene and pentacene. Sincethese organic luminescent materials are provided as single layers havinga thickness of more than 1 μm, a high electric field is required forluminescence. Under in the circumferences, thin film devices formed by avacuum deposition process have been proposed (for example, “Thin SolidFilms” p. 94 (1982); Polymer, 24, 748 (1983); and J. Appl. Phys., 25,L773 (1986)). Although the thin film devices are effective for reducingthe driving voltage, their luminance is far from a level for practicaluse.

[0005] In recent years, Tang et al. has developed an EL device having ahigh luminance for a low driving voltage (Appl. Phys. Lett., 51, 913(1987) and U.S. Pat. No. 4,356,429). The EL device is fabricated bydepositing two significantly thin layers, that is, a charge transportlayer and a luminescent layer, between the anode and the cathode by avacuum deposition process. Such layered organic EL devices are disclosedin, for example, Japanese Patent Laid-Open Nos. 59-194393, 3-264692, and3-163188, U.S. Pat. Nos. 4,539,507 and 4,720,432, and Appl. Phys. Lett.,55, 1467 (1989).

[0006] Also, an EL device of a triple-layered-structure havingindependently a carrier transport function and a luminescent ability wasdisclosed in Jpn. J. Apply. Phys., 27, L269 and L713 (1988). Since thecarrier transportability is improved in such an EL device, theversatility of possible dyes in the luminescent layer is considerablyincreased. Further, the device configuration suggests feasibility ofimproved luminescence by effectively trapping holes and electrons (orexcimers) in the central luminescent layer.

[0007] Layered organic EL devices are generally formed by vacuumdeposition processes. EL devices having considerable luminance are alsoformed by casting processes (as described in, for example, ExtendedAbstracts (The 50th Autumn Meeting (1989), p. 1006 and The 51st AutumnMeeting (1990), p. 1041; The Japan Society of Applied Physics).Considerably high luminance is also achieved by a single-layeredmixture-type EL device, in which the layer is formed byimmersion-coating a solution containing polyvinyl carbazole as a holetransport compound, an oxadiazole derivative as an electron transportcompound and coumarin-6 as a luminescent material (as described inExtended Abstracts (The 38th Spring Meeting (1991), p. 1086; The JapanSociety of Applied Physics and Related Societies).

[0008] As described above, the organic EL devices have beensignificantly improved and have suggested feasibility of a wide varietyof applications; however, these EL devices have some problems forpractical use, for example, insufficient luminance, a change inluminance during use for a long period, and deterioration by atmosphericgas containing oxygen and humidity. Further, it is hard to say that theEL devices sufficiently satisfy needs of diverse wavelengths ofluminescent light for precisely determining luminescent hues of blue,green and red colors in full-color displays etc.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide a luminescentdevice having high output luminance for solving the above-mentionedproblems.

[0010] It is another object of the present invention to provide aluminescent device having excellent durability.

[0011] It is a further object of the present invention to provide aluminescent device easily produced at relatively low production costs.

[0012] A luminescent device in accordance with the present inventionincludes an anode, a cathode, and an organic layer containing at leastone organic compound provided therebetween, wherein the work function ofthe anode Wf(anode) and the Fermi level of the organic layer (Ef(anode))being in contact with the anode satisfies the following equation (I):

Ef(anode)−0.2≦Wf(anode)≦Ef(anode)+0.2[eV]  (I)

[0013] Preferably, the work function of the cathode (Wf(cathode)) andthe Fermi level of the organic layer (Ef(cathode)) being in contact withthe cathode satisfy the following equation (II):

Wf(cathode)≦Ef(cathode)[eV]  (II)

[0014] The luminescent device satisfying the equation (I) or (II) canemerge light with significantly high luminance by a low applied voltageand has excellent durability.

[0015] A luminescent device with a large area can be easily formed by avacuum deposition or casting process with low production costs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a cross-sectional view of an embodiment of a luminescentdevice in accordance with the present invention;

[0017]FIG. 2 is a cross-sectional view of another embodiment of aluminescent device in accordance with the present invention;

[0018]FIG. 3 is a cross-sectional view of a further embodiment of aluminescent device in accordance with the present invention;

[0019]FIG. 4 is a cross-sectional view of still another embodiment of aluminescent device in accordance with the present invention; and

[0020]FIG. 5 is a graph illustrating changes in accumulated charge andintensity of liminescent light with the work function of the metal inthe anode in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0021] The present inventors have studied intensively towards theresolution of the above-mentioned problems, and have discovered apreferable luminescent device including an anode, a cathode, and anorganic layer containing at least one organic compound providedtherebetween, the work function of the anode (Wf(anode)) and the Fermilevel of the organic layer (Ef(anode)) being in contact with the anodesatisfying the following equation (I) and more preferably the equation(II):

Ef(anode)−0.2≦Wf(anode)≦Ef(anode)+0.2[eV]  (I)

Wf(cathode)≦Ef(cathode)[eV]  (II)

[0022] The present invention is completed under such findings.

[0023] In the present invention, the work functions of the anode and thecathode, Wf (anode) and Wf(cathode), respectively, are determined fromthresholds of photoelectron emission with a surface analyzer AC-1 madeby Riken Keiki Co., Ltd. The Fermi levels of the organic layer Ef(anode)and Ef(cathode) are determined by a contact potential method (Kelvinmethod) with a Fermi level meter FAC-1 made by Riken Keiki Co., Ltd.These measurements are performed in an atmospheric condition at atemperature of 20° C. and a humidity of 40%.

[0024] The luminescent device in accordance with the present inventionwill now be described in detail with reference to the drawings.

[0025]FIG. 1 is a schematic cross-sectional view of an embodiment of theluminescent device in accordance with the present invention. An anode 2,a luminescent layer 3 and a cathode 4 are formed on a substrate 1 inthat order. In such a configuration, a usable luminescent layer 3 isgenerally composed of a single compound having hole transportability,electron transportability and luminescence, or a mixture of compoundseach having one of these properties.

[0026]FIG. 2 is a schematic cross-sectional view of another embodimentof the luminescent device in accordance with the present invention. Ananode 2, a hole transport layer 5, an electron transport layer 6 and acathode 4 are formed on a substrate 1 in that order. The hole transportlayer 5 and the electron transport layer 6 function as a luminescentlayer 3. In such a configuration, a usable hole transport layer 5 isgenerally composed of a luminescent material having holetransportability or a mixture including such a material and anon-luminescent material having hole transportability. The luminescentand non-luminescent materials may also have electron transportability.The electron transport layer 6 may be composed of a luminescent materialhaving electron transportability or a mixture including such a materialand a non-luminescent material having electron transportability. Theluminescent and non-luminescent materials may also have holetransportability.

[0027]FIG. 3 is a schematic cross-sectional view of a further embodimentof the luminescent device in accordance with the present invention. Ananode 2, a hole transport layer 5, a luminescent layer 3, an electrontransport layer 6 and a cathode 4 are formed on a substrate 1 in thatorder. FIG. 4 is a schematic cross-sectional view of a still furtherembodiment of the luminescent device in accordance with the presentinvention. An anode 2, a luminescent layer 3, an electron transportlayer 6, and a cathode 4 are formed on a substrate 1 in that order. Inthese configurations, carrier transport and luminescence are performedin the individual layers. Such configurations permit a wide variety ofcombinations of a material having excellent hole transportability, amaterial having excellent electron transportability and a materialhaving excellent luminescence. Further, the configurations permit theuse of various compounds emitting light with different wavelengths;hence the hue of the luminescent light can be controlled within a widerange. Trapping effectively holes and electrons (or excimers) in thecentral luminescent layer will increase the luminescent efficiency.

[0028] The luminescent device in accordance with the present inventionhas excellent hole injection and electron injection characteristicscompared with conventional luminescent devices, and can have theconfigurations shown in FIGS. 1 to 4.

[0029] Organic EL devices generally belong to carrier injectionluminescent devices and their luminance greatly depends on the number ofcarriers, that is holes or electrons, injected from the relevantelectrode. It is preferable that the whole number of the carriersinjected from the anode or cathode be constant in use for a long time.The existing devices, however, deteriorate and cause imperfectelectrical or physical matching between the electrodes and the relevantlayer(s) in use for a long time. As a result, the output luminancesignificantly decreases because of a reduced number of injected carriersin the device.

[0030] The luminescent device satisfying the equation (I) and preferablyalso satisfying the equation (II) in accordance with the presentinvention shows an optimized electrical matching between the electrodesand the adjoining organic layer. A significantly large number ofcarriers are therefore injected through the electrodes even when in usefor a long time, and a reduction in current flow through the device canbe suppressed as much as possible after long-term use.

[0031] The number of carriers injected from the electrodes of a seriesof luminescent devices in accordance with the present invention isobserved as follows. These luminescent devices have the same layerconfiguration as in FIG. 4. Each cathode is composed of aluminum. Theluminescent layer is composed of Alq₃ represented by the followingformula:

[0032] Each hole transport layer is composed of the compound representedby the following formula (a) and a polycarbonate resin (1:1 mixture byweight ratio):

[0033] Each anode is composed of any one of Al, Fe, Pd, Ag, Au, Ni andCu, which have different work functions. The anode was formed so that itwas translucent. The number of holes injected from the anode to the holetransfer layer and the intensity of the luminescent light weredetermined. The results are shown in the graph of FIG. 5. The horizontalaxis of the graph represents the work function of the metal used in theanode. The work function was determined using a surface analyzer AC-1made by Riken Keiki Co., Ltd. The accumulated charge by injectedcarriers at the vertical axis was determined with a spatial chargedistribution analyzer PEANUTS made by Five Labs. Inc. The Fermi level ofthe hole transport layer determined with a Fermi level meter FAC-1 madeby Riken Keiki Co., Ltd. was 4.52 eV.

[0034]FIG. 5 demonstrates that an anode having a work function within arange of ±0.2 eV from the Fermi level (4.52 eV) of the hole transportlayer has an accumulated charge (a number of the injected holes) whichis approximately 70% of the maximum value. The intensity of theluminescent light depends on the number of the injected holes. It ispreferable that the anode has a work function within a range of ±0.1 eVfrom the Fermi level of the hole transport layer in order to achieve alarger intensity of the luminescent light. Since in such a luminescentdevice a large number of carriers are injected, the luminescent lighthas a large intensity and the decrease in current flow in the device isnot remarkable after long-term use.

[0035] A slightly different phenomenon was observed between the electrontransport layer and the cathode. A window accepting electrons from thecathode widely spreads near the Fermi level of the organic compoundhaving electron transportability, hence the number of the injectedcarriers (electrons) and thus the luminescent intensity increase as thework function of the cathode becomes lower than the Fermi level of theelectron transport layer.

[0036] As components of the luminescent layer in the luminescent devicein accordance with the present invention, hole transport materialsstudied in the field of electrophotographic photosensitive members andknown luminescent hole transport materials as shown in Tables 1 to 4 orelectron transport materials and known luminescent electron transportmaterials as shown in Table 5 to 6 can be used. These materials are usedalone or in combination. TABLE 1 Hole Transport Compounds

[0037] TABLE 2 Hole Transport Compounds

[0038] TABLE 3 Hole Transport Compounds

[0039] TABLE 4 Luminescent Hole Transparent Compounds

[0040] TABLE 5 Electron Transport Compounds

[0041] TABLE 6 Luminescent Electron Transport Compounds

[0042] In the luminescent device in accordance with the presentinvention, the luminescent layer is generally formed by a vacuumdeposition process or using a binding resin.

[0043] Non-limiting examples of the binding resins include polyvinylcarbazole resins, polycarbonate resins, polyester resins, polyarylateresins, butyral resins, polystyrene resins, polyvinyl acetal resins,diallyl phthalate resins, acrylate resins, methacrylate resins, phenolresins, epoxy resins, silicon resins, polysulfone resins, and urearesins. These binding resins can be used alone or in combination.

[0044] Preferable anode materials have large work functions. Examples ofsuch materials include nickel, gold, platinum, palladium, selenium,rhenium, and iridium; alloys thereof; and tin oxide, indium tin oxide,and copper iodide. Conductive polymers, such as poly(3-methylthiophene),polyphenylene sulfide and polypyrrole are also usable.

[0045] In contrast, preferable cathode materials have small workfunctions. Examples of such materials include silver, lead, tin,magnesium, aluminum, calcium, manganese, indium and chromium, and alloysthereof.

[0046] It is preferable that at least one constituent of the anode andcathode transmits 50% or more of incident light over the wavelengthregion of the luminescent light.

[0047] As the transparent substrate, glass and plastic films are used inthe present invention.

[0048] The luminescent device in accordance with the present inventionis a thin, light and solid device with a large luminescent area and highresolution, enabling high-speed operation. Such a luminescent device isquite different from conventional incandescent lamps, fluorescent lamps,and light emitting diodes, and can be used for electroluminescencepanels which are expected as devices satisfying advanced needs inindustrial fields.

EXAMPLES

[0049] The present invention is described in further detail withreference to the following examples.

[0050] Examples 1 to 3 Comparative Examples 1 and 2

[0051] Luminescent devices of Examples 1, 2 and 3 were prepared asfollows. Translucent or transparent anodes with a thickness of 50 nmcomposed of iron, silver and indium tin oxide (ITO), respectively wereformed on glass plates by a deposition process. On each anode, a holetransport layer with a thickness 100 nm of the following compound (b),an electron transport layer with a thickness of 75 nm composed of Alq₃,and a cathode with a thickness of 120 nm composed of aluminum wereformed in that order by a vacuum deposition process. The electrontransport layer also functions as a luminescent layer.

[0052] Luminescent devices of Comparative Examples 1 and 2 were alsoprepared as in Example 1, but the anode materials were zirconium andcopper, respectively, and the cathode was composed of gold and had athickness of 100 nm.

[0053] Table 7 shows the results of the work functions Wf (eV) of theanodes and the cathodes determined with the above-mentioned surfaceanalyzer AC-1.

[0054] The Fermi levels of the films composed of the compound (b) andAlq₃ determined with the above-mentioned Fermi level meter FAC-1 were4.63 eV and 4.70 eV, respectively.

[0055] A current flow of a current density of 20 mA/cm² was applied tothe luminescent devices for 100 hours. The results are also shown inTable 7. TABLE 7 Initial After 100 hours Cathode Anode AppliedLuminescent Applied Luminescent Wf Wf Voltage Output Voltage OutputSample Metal (eV) Metal (eV) (V) (μW/cm²) (V) (μW/cm²) Example 1 Al 4.08Fe 4.43 19 30 32 30 Example 2 Al 4.08 Ag 4.71 13 40 17 37 Example 3 Al4.08 ITO 4.66 10 44 13 41 Comp. Ex 1 Au 4.71 Zn 4.26 50 None 64 NoneLuminescence Luminescence Comp. Ex. 2 Au 4.71 Cu 5.08 55 None 77 NoneLuminescence Luminescence

[0056] Example 4 and 5, and Comparative Example 3 and 4

[0057] Luminescent devices of Examples 4 and 5 were prepared as follows.Translucent or transparent anodes with a thickness of 50 nm composed ofITO and molybdenum, respectively were formed on glass plates by adeposition process. On each anode, a hole transport layer with athickness 130 nm of the following compound (c), an electron transportlayer with a thickness of 100 nm composed of titanyloxyphthalocyanine,and a cathode with a thickness of 120 nm composed of aluminum wereformed in that order by a vacuum deposition process. The electrontransport layer also functions as a luminescent layer.

[0058] Luminescent devices of Comparative Examples 3 and 4 were alsoprepared as in Example 4, but the anode materials were zinc and nickel,respectively, and the cathode was composed of platinum and had athickness of 125 nm.

[0059] Table 8 shows the results of the work functions Wf (eV) of theanodes and the cathodes determined with the above-mentioned surfaceanalyzer AC-1.

[0060] The Fermi levels of the films composed of the compound (c) andtitanyloxyphthalocyanine determined with the above-mentioned Fermi levelmeter FAC-1 were 4.56 eV and 4.94 eV, respectively.

[0061] A current flow of a current density of 60 mA/cm² was applied tothe luminescent devices for 100 hours. The results are also shown inTable 8. TABLE 8 Initial After 100 hours Cathode Anode AppliedLuminescent Applied Luminescent Wf Wf Voltage Output Voltage OutputSample Metal (eV) Metal (eV) (V) (μW/cm²) (V) (μW/cm²) Example 4 Al 4.08ITO 4.66 30 4.0 35 3.3 Example 5 Al 4.08 Mo 4.36 38 3.0 48 1.9 Comp. Ex3 Pt 5.20 Zn 4.13 88 None 108  None Luminescence Luminescence Comp. Ex.4 Pt 5.20 Ni 4.84 80 None 98 None Luminescence Luminescence

[0062] While the present invention has been described with reference towhat are presently considered that the invention is not limited to thedisclosed embodiments. To the contrary, the invention is intended tocover various modifications and equivalent arrangements, included withinthe spirit and scope of the appended claims. The scope of the followingclaims is to be accorded the broadest interpretation so as to encompassall such modifications and equivalent structures and functions.

What is claimed is:
 1. A luminescent device comprising an anode, acathode, and an organic layer containing at least one organic compoundprovided therebetween, wherein the work function of the anode(Wf(anode)) and the Fermi level of the organic layer (Ef(anode)) beingin contact with the anode satisfies the following equation (I):Ef(anode)−0.2≦Wf(anode)≦Ef(anode)+0.2[eV]  (I)
 2. A luminescent deviceaccording to claim 1, wherein the work function of the cathode(Wf(cathode)) and the Fermi level of the organic layer (Ef(cathode))being in contact with the cathode satisfies the following equation (II):Wf(cathode)≦Ef(cathode)[eV]  (II)